The transport of buoyant particles through a conduit to a submerged location in a fluid presents a complex and challenging fluid dynamics problem. As an initial consideration, when the buoyant particles are mixed with a transport fluid in the conduit, the buoyant force acting on the particles must be overcome by a greater downward fluid dynamic drag force on the particles. Also, the static pressure head created by the fluid in the vessel must be overcome. The value of the static pressure is directly related to the density of the fluid in the vessel and the depth to which the buoyant particles are being transported. Beyond these considerations, however, one must take into account the behavior of the buoyant particles/fluid mixture in the pipe in terms of (i) loss of entrainment of particles in low velocity portions of the flow field which may be caused by the effect of the pipe walls on the flow and (ii) the "N body problem," i.e., the effect of the relative motion of many bodies being propelled by fluid flow in close proximity to one another. All of the above considerations present a fluid dynamics problem that is not one subject to facile analysis. As one basic proposition, in order to maintain an acceptable velocity of the buoyant particles in the delivery conduit, the aggregate axial drag on the particles must exceed the aggregate buoyant force on the particles as measured over the entire path of travel through the conduit. Also, the dynamic pressure head of the buoyant particles/fluid mixture as it is introduced into the delivery pipe must be sufficient to overcome the static pressure head plus frictional losses at the boundary. At any particular mass flow rate, pipe diameter and delivery depth, the possibility of loss of entrainment and the N body problem must also be taken into account.
The above situation may be addressed by the use of "brute force"; i.e., pumping the buoyant particles/fluid mixture into the delivery conduit with a high velocity and high kinetic energy sufficient to push the mixture through the conduit. However, this approach suffers the disadvantage of being wasteful from an energy standpoint and requires the use of expensive and complex pumps for supplying the necessary energy to a mixture of buoyant particles and fluid.
Another approach of a different nature simply uses a mechanical conveyance, such as an auger screw pump. But, for many applications, it is undesirable to have mechanical means that may be subject to repair or maintenance at a submerged location.
Other solutions include the use of an aspirating jet pump, as discussed, for example, in commonly owned, co-pending application Ser. No. 07/543,360 entitled "Thermal Storage Tank System and Method", now U.S. Pat. No. 5,063,748. While the aspirating pump may serve to entrain the buoyant particles in the fluid by providing sufficient drag on the buoyant particles so that the drag on the particles exceeds their buoyancy, it may not maintain the desired entrainment once "fully developed pipe flow" velocity profiles are established. Beyond this point, buoyant particles will migrate to the low velocity field near the conduit wall and may even reverse direction of movement if the fluid dynamic drag becomes less than the buoyant force. Furthermore, a low velocity field may result in a bridging phenomenon caused by ice particle agglomeration which may be especially acute when the transport water is at or near 32.degree. F.
In view of the above, it will be appreciated that there is a need for an energy efficient, reliable and simple system for addressing the above problems with respect to the delivery of buoyant particles to a submerged location in a flooded vessel and, as discussed immediately below, particularly with respect to the delivery of ice to a submerged location in a thermal storage tank.
Recently, the long term storage of thermal energy has emerged as a means to spread out electricity usage over long periods of time to reduce electric utilities' short term seasonal loads. According to one long term thermal energy storage system, an ice machine is operated on a continuous basis year-round with the ice being stored in an ice/water mixture in a thermal storage tank. The thermal energy in the tank is available for use on demand during periods that coincide with the electric utility's seasonal peak load operating times during the summer, for example, to provide space air-conditioning, harvested crop cooling, greenhouse space cooling, poultry house cooling, etc. Thus, the system enables the electric utility to substitute a relatively small, long term level load for a much higher direct acting load that would coincide with, or largely overlap, the utility's seasonal peak usage.
The above system requires a thermal storage tank that will efficiently store the generated ice over long term periods measured in months. It has been found desirable in the operation of such a thermal storage tank to maintain the tank in a flooded state such that it is continuously filled with an ice/water mixture. After a long period of charging the tank with ice, the ice fraction in the tank will increase to a high level, for example, 85% or higher. In contrast, after a substantial amount of the ice has been melted to meet short term cooling demands, the ice friction may be as little as, for example, 5-15%.
It has been found desirable in the operation of such thermal energy storage systems to deliver the ice through a conduit to the flooded thermal storage tank at a point near the bottom of the tank so that the ice mass "grows" in the tank in the shape of an inverted cone. However, the delivery of the ice particles to a submerged location in the thermal storage tank presents the fluid dynamics problem that is discussed above--a problem that may be exacerbated by the tendency of the ice particles to stick together or "agglomerate" during transport under certain temperature conditions, i.e., subfreezing surface temperatures. Thus, there is a need for a delivery system of the mentioned class, particularly suited to the delivery of ice particles.
While ice delivery is one important environment in which the above buoyant particle transport system is applicable, other environments lend themselves to this approach as well, for example, processes in the food industry, pulp and paper industry, and chemical processing industry in which it is desirable to inject buoyant particles at the bottom of a bath or reactor tank followed by recovery of the particles after they have floated to the surface.